Pseudomonas aeruginosa (PA) is an opportunistic bacterial pathogen responsible for a number of clinically important lung diseases including pneumonia and cystic fibrosis (CF). In the case of CF, the major cause of morbidity and mortality among afflicted patients is airway obstruction due to the presence of thick and tenacious mucus that becomes heavily infected with PA. Because PA exposure occurs in the respiratory system of both normal and CF individuals, "selective"' infection by this pathogen among CF patients suggests the presence of a disease-causing mechanism that is not present in non-CF airways. A number of different theories have been proposed to account for the etiology of CF. Our laboratory made the interesting observation that MUC1 mucin on the surface of airway epithelial cells is a specific binding site for PA mediated through bacterial flagellin. The structure of the MUC1 glycoprotein suggests that it acts as a receptor to transmit signals intracellularly following interaction with flagellin. Using mice genetically modified to block MUC1 expression (MUC1 knockout mice), our preliminary studies showed that, compared with wild type mice, Muc1 knockout animals exhibited increased PA clearance from the lungs and greater recruitment of airway leukocytes and higher levels of the proinflammatory cytokines in bronchoalveolar lavage fluid following PA flagellin stimulation. Interestingly, TLR5 is another cell surface receptor that generates an intracellular signaling pathway following binding to flagellin. Based on this similarity, we conducted additional experiments to investigate the functional relationship between MUC1 and TLR5. We observed that expression of MUC1 inhibited the flagellin-TLR5 signaling pathway in normal lung cells but not CF airway epithelial cells. Based on these results, we formed the hypothesis that MUC1 is an anti-inflammatory cell surface receptor that acts, at least in part, through antagonism of flagellin-TLR5 signaling. In this proposal, we will test our theory by determining the mechanisms by which MUC1 attenuates TLR5 signal transduction. Successful completion of this project will provide important insights for the role of MUC1 in inflammation, innate immunity, and the early stages of PA infection in CF. [unreadable] [unreadable] [unreadable]